Wood-based composites are widely used in North America in both structural and non-structural applications. Structural applications include use of these materials for framing and sheathing (floors, walls and roofs) in residential construction. One of the most common composites used for these applications is oriented strandboard (OSB).
OSB is typically manufactured in seven major stages, which include stranding, screening, drying, blending, forming, pressing, and finishing. Stranding is a process in which logs are cut into discrete wooden strands (also known as flakes) that typically have an aspect ratio between 1 and 20. The long axis of the strands is predominantly aligned within about 0-30° of the grain of the wood. The strand thickness can range from about 0.015″ to about 0.050″ and the length can range from about 1″ to about 12″, or even longer. In most cases, it is advantageous to cut strands from wood that has a relatively high moisture content, such as 30-200%. Thus, the stranding process almost always yields wet strands that must be dried prior to further processing. Drying is usually accomplished by passing the wet strands through long rotating drums or pipes in conjunction with hot, dry air. Alternatively, strands can be dried by conveying them in a chamber with hot, dry air passing through or around the conveyance system. The drying process commonly results in strands that have a moisture content of about 1-7%. The resulting dried strands exist as a mixture of relatively large and small elements, and it is frequently desirable to screen the material in order to separate the strands on the basis of size. In many cases the smallest wooden particles, known as fines, are diverted from the product stream and are transported to a burner where they are used as fuel. The acceptable wooden strands are then metered into large rotating drums, known as blenders, and are sprayed or otherwise mixed with bonding resin (adhesive) and wax. This stage is known as blending. Many blenders rotate at a rate of about 4-20 rpm and are tilted)(3-8° in order to promote material flow. A single blender can have about six liquid application devices distributed within it. Such application devices are frequently rotary disk atomizers, but they can also be simple spray guns. In some cases one of the application devices is devoted to wax and five of the application devices are devoted to resin. Powdered bonding resins can also be introduced into the blender. It is common for large strands and small strands to be blended separately. Strands that have been treated with bonding resin and wax are then formed into a mat. Treated strands are formed into a mat by dispensing them at some controlled rate through mechanical partitions that tend to align the strands in a particular orientation. As the strands pass through the alignment devices they are collected onto a continuous conveyor belt. Frequently, the strands incorporated into the top and bottom layers of an OSB mat are larger than the strands incorporated into the core (or middle) layer of the mat. In many cases the bonding resins and application levels used in the surface layers of an OSB mat are different than those used in the core layer of the mat. In a commercial manufacturing process the mat is generally continuous in length and has a width of between about 4′ and 16′. The thickness of the mat can be in the range of about 2″ to 20″. In some cases the continuous mat of treated strands is transported directly into a continuous hot-press, but in most cases the mat is cut into discrete sections, which commonly have a length of about 8′ to 20′. These mat sections are loaded into a multi-opening hot press, which can usually press between 12 and 20 mats simultaneously. During the process of pressing strands are forced together and intimate contact is achieved along their interface. Subsequent to this consolidation process bond formation occurs as the resin undergoes curing reactions and is converted from a liquid to a load-bearing solid. The press then opens and the relatively large “jumbo” panels are ejected onto a conveyor and transported to the finishing stages of the operation. Finishing steps commonly include cutting the jumbo panels into smaller panels, such as those having dimensions of 4″×8″. Other finishing activities can include sanding, edge profiling, marking with grade stamps, grading for quality, stacking into units, sealing, labeling, strapping and packaging.
It is common for OSB panels to be manufactured by use of PMDI (polymeric methylene diphenyldiisocyanate) as a core-layer adhesive and PF (phenol/formaldehyde) resin as a surface layer adhesive. The PMDI is relatively expensive and it bonds to metal press platens. Thus, PMDI is not usually used as a surface layer adhesive, but it is associated with significantly improved water absorption rate when the resulting OSB is exposed to precipitation during the construction process. The PMDI has the further advantage of resulting in increased bond strength between adjacent strands, as compared to PF adhesive. Thus, many OSB manufacturers use PMDI as the adhesive in the core layer as opposed to PF resin.
In order to achieve even lower water absorption rate, a few OSB manufacturers have replaced PF adhesive in the surface layer of the panel with PMDI. This results in an OSB panel with exceptional water resistance. A further benefit to this practice is that the panel retains a relatively light-color when it is exposed to water, as opposed to a dark, brown color which is commonly experienced by OSB that is made with a PF adhesive in the surface layer and is subsequently exposed to water. Unfortunately, OSB manufacturers who utilize PMDI in the surface layer must exert significant effort to ensure that the PMDI-treated wooden strands do not bond to the steel platens in the hot-press. Typically, prevention of press sticking is achieved by application of release agents to the top and bottom of the strand mat just prior to hot-pressing. Unfortunately, the release agents (usually tall oil fatty acid soaps or wax emulsions) are relatively expensive and they tend to darken the surface of the OSB, which lessens the value of the panel in the marketplace. The release agents can also accumulate on the press platens and some of them promote corrosion of the press platens. Thus, most OSB manufacturers choose not to use PMDI in the surface layer of the OSB, although it would be beneficial to panel properties.
Thus, there is a need for an alternative surface layer adhesive for OSB and other wood-based composites that achieves a light-colored surface, reduced water absorption rate, and is associated with less sticking to the steel press platens during the manufacturing process.